This invention relates to a method of cation (e.g., metal) reduction by anode-assisted electrolysis.
The total potential E(total) in volts of a practical electrowinning cell may be given by: EQU E(total)=E.sub.A -E.sub.c +E(op)+iR
where E.sub.A is the potential of the anodic reaction H.sub.2 0.fwdarw.1/20.sub.2 +2H.sup.+ +2e, E.sub.c is the potential for reducing the metal ion or hydrogen ion (at the cathode), E(op) includes the associated overpotentials, and iR is the potential drop within the circuit of resistance R (ohms) carrying a current i (amps). When the oxygen pressure is at one atmosphere and a.sub.H +=1, i.e., pH=O, E.sub.A becomes E.degree..sub.A of value 1.23 V at 25.degree. C.
Metal reduction by anode-assisted electrolysis has been described by Farooque and Coughlin (Nature, August 23, 1979), who propose that carbon should be provided as a reducing agent at the anode, so that the anodic reaction becomes (they say) EQU C+2H.sub.2 O.fwdarw.CO.sub.2 +4H.sup.+ +4e
for which E.degree..sub.A is only about 0.21 V. This substantially lessens E(total). Farooque and Coughlin propose to provide the carbon in the form of a coal or lignite slurry agitated against a platinum mesh anode, for their anode-assisted metal reduction. However using this method we find that frequent rest periods are necessary to keep the anode at peak effectiveness, unless the anode current density is kept down to about 20 Am.sup.-2, which is far too low for industrial acceptability.
Report No. 1754 (June 1975) of the National Institute for Metallurgy, South Africa, suggests that ferrous ion in a concentration of 50 to 55 g/l could be used as a reducing agent at the anode, with techniques to enhance mass transfer to the anode surface, the anode consisting of a packed bed of, for example, graphite grains to minimize the current density per unit area of the anode.
This ferrous ion concentration is so high as to interfere with the electrowinning reduction at the cathode unless a diaphragm is provided between anode and cathode. A diaphragm is one of the more troublesome components of a cell.